This chapter is from the book

Cocoa is a collection of object-oriented components written in the
Objective-C language. Objective-C is a flexible and dynamic language that adds
object-oriented extensions to ANSI standard C. Because of the flexibility of
Objective-C, the Cocoa components can be used by a wide variety of languages
besides Objective-C. The key language elements needed to use Cocoa are support
for dynamic object orientation and compatibility with the C programming
language.

This chapter describes the general features of all languages that can be used
with Cocoa, and a brief overview of object-oriented terminology. Details about
the most common languages used with Cocoa are provided. The available language
options are explained along with some of the advantages and disadvantages of
different languages. This book primarily uses the Objective-C language, and this
chapter explains why.

Java and many scripting languages provide the required language features to
use Cocoa directly. Other languages such as C and C++ are not sufficiently
dynamic to use Cocoa objects directly. C and C++ programs can access Cocoa in
two ways: They can use Cocoa indirectly via the C-based Objective-C runtime, or
they can be recompiled using the Objective-C and Objective-C++ language
compilers.

Details about using the Objective-C runtime from C or C++ without using the
Objective-C language syntax are provided in Appendix A, "Unleashing the
Objective-C Runtime." Because Objective-C is an extension of standard C and
can compile all C programs, the best way to use Cocoa from C code is to actually
use Objective-C. Parts of a program can be standard C (perhaps for portability
reasons), whereas other parts can use Objective-C's object-oriented
extensions to access Cocoa. A variant of Objective-C called Objective-C++
provides the same object-oriented extensions to C++ that Objective-C provides to
standard C. The best way to mix C++ code and Cocoa is to use Objective-C++.

Object Orientation

Cocoa is a collection of objects. To understand how various languages use
Cocoa, a brief explanation of objects and object orientation is needed.
Object-oriented languages must use objects in a way that is compatible with
Cocoa to be integrated with Cocoa. For example, Java is an object-oriented
language that interfaces well with Cocoa. C++ is an object-oriented language
that provides a model of objects incompatible with Cocoa.

The goals of object orientation are to make writing software easier, cheaper,
and faster. The principal way that object orientation achieves its goals is
through software reuse. The idea is that software can be organized into objects
that are individually reusable. Each time a new software project begins,
substantial parts of the project can be implemented using existing software
objects. In theory, the only new code that is written for a project is the code
that is truly unique to that project and cannot be shared by other projects.

By reusing objects, programmers hope to reduce the amount of new code written
for each project and, therefore, finish the project faster. Reused objects are
already well-tested in other projects. By reusing objects, programmers avoid
bugs that might be created in new code. Existing objects that implement complex
logic are used to make creating software for a new project easier. The idea is
that the most complex and difficult to write code is provided by existing
objects. Reusing objects is simpler than rewriting the logic implemented by the
objects.

Encapsulation

Encapsulation is one of the principal ideas of object orientation.
Encapsulation means that data and the logic that operates on the data are
grouped together. Data is only modified via the operations encapsulated with the
data. Encapsulation aids reuse and simplifies software maintenance.
Encapsulation also ensures that related data and logic can be easily identified
in one project and reused in another. Because data and logic are encapsulated
together, it is not necessary to grab a line of code here and a data structure
definition there, or search all the source code in a project for necessary lines
of code to reuse just one feature of a project. Encapsulation aids software
maintenance by localizing problem solutions. If a bug is detected, or a new
feature must be added, encapsulation ensures that there is only one place in the
code where changes are made. Without encapsulation, fixing a bug, or adding a
feature might require changes to many different parts of a software project.

Modularity

Modularity is related to encapsulation. Modularity is the practice of
implementing a software project in multiple modules. A module is a
self-contained input to a compiler. The idea is that modules of code and data
can be developed and compiled independently. The separately compiled modules are
brought together to complete a project. In many cases, each module encapsulates
some data and logic. Apple's Objective-C compiler enables the use of
multiple modules to encapsulate one set of data and operations. It is also
possible to implement multiple units of encapsulation in one module, but it is
usually a bad practice.

Classes

In most object-oriented languages, something called a class is the basis of
encapsulation. Each class encapsulates some data and all operations upon that
data. Operations on data are sometimes called behaviors. Classes are implemented
in one or more modules. Classes formalize the ideas of encapsulation, and in
some languages the compiler enforces that encapsulation. The compiler prevents
code that is not part of a class from modifying the data managed by the class.
Classes are related to instances and inheritance.

Instances

A class is in some respects an abstract idea. A class describes data and
operations on that data, but a class by itself is usually just an idea. For
example, imagine a class called Control that describes certain
characteristics of all graphical user interface elements. Those characteristics
might include color and position. The class Control is not any
particular user interface element, it describes all user interface elements. A
particular button is an "instance" of the class Control. An
instance of the class Control has a color and a position as described
by the Control class.

A class describes data and behavior. An instance actually stores the data
described by a class. There can be any number of instances of a class. The
behaviors defined in a class are applied to instance's data.

NOTE

You will learn more about abstract classes and abstract Cocoa classes in
Chapters 4 and 7. Chapter 4 also introduces the details of Objective-C as well
as class methods versus instance methods. If these concepts are confusing to you
now, you'll get more details in these two chapters.

Objects

Both classes and instances can be called objects. Classes are objects that
describe instances. Instances are objects that store data described by a class.
Object is a general term that applies to encapsulated data and logic and has
different implications in different languages. Almost every object-oriented
language endows objects with capabilities beyond just encapsulation, such as
support for specialization.

Specialization

Object orientation promotes software reuse. Software objects from one project
can be used in another project. But what happens when a new project needs an
object similar to one that already exists, but needs just a few changes? The
existing object can be specialized rather than starting from scratch to create a
new object that meets the exact needs of the new project. Specialization is a
technique for altering parts of the data and behavior of an existing object
while reusing other parts. There are two types of specialization: instance-based
and class-based. Cocoa uses both class-based specialization and instance-based
specialization extensively.

Instance-based specialization is a technique for changing the behavior of
just one instance object without necessarily modifying the behavior of other
instances of the same class.

Class-based specialization applies changes to classes. The most common
technique is to create a new class that includes all the data and operations of
another class while adding additional data and new behaviors. Instances of the
new class store the additional data and have the additional behaviors along with
the data and behaviors of instances of the original class.

Inheritance

Inheritance is the most common form of class-based specialization. If a class
called Button includes the data and behaviors of class
Control, class Button is said to inherit from class
Control. The terms subclass and superclass describe the inheritance
relationship. Button is a subclass of Control. A subclass
inherits all of the data and behavior of its superclass. In this example,
Control is Button's superclass. If class Button
inherits from class Control, an instance of Button can be used
in any context that an instance of Control is required.

Some languages such as C++ support multiple inheritance. Multiple inheritance
means that a class has all the data and behavior described by two or more super
classes. Java and Objective-C do not support multiple inheritance, and Cocoa
does not use multiple inheritance.

Messages

Messages are one way that objects can communicate with each other. Messages
enable objects to request that other objects invoke behaviors. A user interface
object might send the isEnabled message to a Control instance
as a way to request that the Control instance returns a YES or
NO value. A message is an abstract idea and few assumptions are made
about messages. For example, a message can be sent to an anonymous object. The
sender of the message might not know the class of the receiver. The receiver
might not even be in the same program as the sender. Messages promote object
reuse by minimizing the dependencies between objects. The less one object knows
about another, the better chance the objects can be reused separately. Messaging
enables communication between objects without dependencies.

Many object-oriented languages do not directly support messaging. Messaging
is one of the dynamic features of Objective-C that are essential to the
implementation of Cocoa. Messaging is described in Chapter 4, and the technical
implementation of messaging is described in Appendix A.

Polymorphism

Polymorphism is another technique that enables the reuse of software objects.
Polymorphism allows the software that uses an object to work regardless of the
specific class of the object. In other words, polymorphism enables an object to
send the same message to different receivers without knowing precisely what
behavior will be invoked by the message when it is received.

All messages in Objective-C use polymorphism. In many cases it is not
possible for the sender of an Objective-C message to know the class of the
object that receives the message. Each receiver will invoke different behaviors
upon receipt of the message. Java and C++ also support polymorphism to various
degrees. Along the spectrum of flexibility, C++ compilers require detailed
knowledge about all objects whose behaviors are used. Objective-C does not
require any knowledge about the objects that are used at compile time. Java
falls between the two extremes.